The invention relates to the field of locking devices for paperboard cartons and, more particularly, to a corner lock for a carton exhibiting improved lock retention.
Over the years it has been a goal of carton manufacturers to produce a corner lock for paperboard cartons which is simple to construct and erect using automatic machinery, and which provides a secure interlock between the carton side panels regardless of the forces applied to the lock.
Corner-lock type cartons are well known in the art and are usually folded and erected from a paperboard blank through the use of automatic machinery. Generally, the paperboard blank, which has already been cut and scored, is fed into the carton erecting machine and folded along the score lines to form the carton. In some types of open tray cartons, due to the resiliency of the paperboard, the side panels of the carton do not stand perpendicular to the base panel but appear bowed slightly outward after they are erected. Therefore, the carton folding and erecting procedure sometimes includes a step in which the erected side panels are "over-broken", i.e., folded inwardly beyond an angle perpendicular to the base panel after erection, in order to make the wall panel stand perpendicular to the base panel when pressure is released.
Some form of more secure lock retention means is thus required, especially for this type of corner lock carton. One type of retention means takes the form of an extended portion or tongue of the lock member which is pulled or pushed through the slit. This corner lock is known as a stripper lock. The locking tongue is "stripped" or forced through the slit by means of an arm provided on the carton erecting apparatus. Even as secure as prior art "stripper-type" locking arrangements have been, when the erected side panels are "over-broken" along their crease lines, the projecting portion of the tongue, which acts as the retention means for the lock, still may be accidentally released and slip through the slit, allowing the corner lock to become accidentally disengaged.
One type of corner lock for cartons is illustrated in U.S. Pat. Nos. 2,580,181 and 2,712,409. These patents show an angled locking tongue formed on a corner panel which interengages a complementary slit formed on the adjacent side panel. An edge of the locking tongue and the slit lie parallel to, and are in contact with, one another along their engaging edges so as to distribute forces along the entire length of the engaging edges.
Another type of corner lock is shown in U.S. Pat. Nos. 3,069,061 and 3,226,006 in which a locking tongue having an angled edge is formed on a corner panel and interengages an angled slit formed on a side panel. The angled edge of the tongue and the angled slit are disposed at an angle to each other such that their respective edges cross at a single point midway along their edges.
U.S. Pat. No. 3,037,680 also discloses a corner lock in which an angled edge of a locking tongue also interengages an angled slit formed on a side panel. The slit is disposed at an angle slightly different than the angle of the tongue edge. The edges cross at a point near the juncture of the locking tongue with its corner panel and approximately midway along the angled slit edge.
The above-described types of corner locking arrangements have the disadvantage that if the wall panels are put under stress, especially outwardly directed stress, the point of contact of the engaging edges of the corner lock tend to move, resulting in greater wear and possible failure of the lock while under stress. In addition, the locking tongue can become disengaged from the slit under inwardly directed side panel movement, allowing the corner lock to become disengaged.
Thus it is a primary object of the invention to provide an improved lock for a corner lock-type carton having positive and secure lock retention means.
It is an additional object to provide an improved corner-lock type carton which is readily folded and erected by automatic machinery.
It is another object to provide an improved "stripper-type" lock for a corner lock carton exhibiting reduced wear under stress.
It is a further object to provide an improved lock for a corner lock-type carton which is simple in construction and easily manufactured.